Forgeable high strength austenitic alloy with copper and columbiumtantalum additions



F. 'r. EBERLE 2,797,992 FORGEABLE HIGH STRENGTH AUSTENITIC ALLOY WITH COPPER July 2, 1957 AND COLUMBIUM-TANTALUM ADDITIONS Filed July 50. 1954 mmmmkm mwoz: mmDOI n N mw wnn a INVENTOR in t2 TEfier/e ATTORNEY FORGEABLE HIGH TRENGTH AUSTENITIC ALLOY WHTH CUPPER AND COLUMBIUM- TANTALUM ADDITKONS Fritz 'll. Eberle, Earberton, Ohio, assignor to The Babcock & Wilcox Company, a corporation of New Jersey Application July 30, 1954, Serial No. 446,975

8 Claims. (Cl. 75125) This invention relates to forgeable alloy steels having enhanced stress rupture strength, corrosion resistance, and freedom from embrittlement in extended service, at elevated temperatures and stresses, and, more particularly, to a fully austenitic chrome-nickel-iron alloy steel attaining the foregoing properties with a minimum total alloy content.

For a number of years there has been a steady in crease in the superheater outlet temperatures and pressures of vapor generators, with a resulting increase in the efliciency and economy of turbines driving electric generators. These temperature and pressure increases required alloy steels to be used in the superheaters, such as stainless steels of the columbium and titanium bearing 18Cr-8Ni AISI Types 347 and 321. With superheater outlet temperatures of 1050 F., the pressures involved are frequently substantially in excess of 2000 p. s. i.

With pressures of this order, the superheater tubing must have wall thicknesses of up to A" for such 18-8 alloys to remain within their allowable working stresses. Such wall thicknesses are undesirable, not only from the standpoint of fabrication problems but also from the standpoint of heat transfer and thermal stress gradients across the wall of the tubing. As a consequence, the increase in superheater outlet temperatures recently has been arrested at substantially the 1100 F. level.

Any further substantial increase in superheater outlet temperatures requires steel alloys capable of practical fabrication into tubing having'wall thicknesses acceptable from the fabrication, heat transfer and thermal stress gradient standpoints, and having long-time strength and corrosion resistance at temperatures in excess of 1350 F. and pressures substantially in excess of 2000 p. s. i. In addition, considering the large quantities of such tubing required in modern vapor generator installations, such alloys must have a low total alloy content in order to be economically feasible for use as superheater tubing.

There are known alloys which have long-time strength at high temperatures but which either have too high an alloy content to be economically practical for use as superheater tubing or are substantially non-forgeable, difficult to force, or characterized by a loss of desirable properties in long-time service at elevated temperatures.

The present invention is, accordingly, directed to a steel alloy capable of economically practical use as tubing operating at temperatures in excess of 1350 F. and pressures in excess of 2000 p. s. i., and having the lowest possible alloy content, being particularly low or lean in strategically important elements. The invention is particularly directed to such an alloy meeting the following requirements:

1. Stress-rupture strength, at 1350 F., at least twice that of A181 Type 304 alloys, the most economical steel alloys commercially available for use at such elevated temperatures;

2. Adequate resistance to corrosion by superheated vapor and combustion gases at 1350 F.;

3. Adequate hot plasticity, for fabrication into tubing;

2,797,992 Patented July 2, 1957 Percent Cr 15.00-20.00 Ni 12.00-18.00 C 0.02-0.15 Mn 0.25-2.50 Si 0.10-1.00

Balance iron with the usual impurities.

This base composition is a fully austenitic iron-chromenickel steel alloy of relatively low carbon and silicon content. The chromium content is sufficiently high for adequate oxidation and corrosion resistance at temperatures of the order of about 1500 F, and yet sufiiciently low to suppress sigma-phase formation. The nickel content is sulficient to maintain the alloy structure fully austenitic over a wide range of variation in alloying additions. The fully austenitic, or face-centered lattice, structure is important for maximum sustained high-temperature strength, the low carbon content assures hot plasticity and weldability, and the low silicon content is adequate insurance against micro-issuring in welding.

The creep-rupture .strengthof this base composition is raised by suitable alloy addition designed to produce age hardening processes in the base composition by forming complex carbides or intermetallic compounds which are soluble in the base composition at very high temperatures but insoluble or of limited solubility therein at lower temperatures in the general vicinity of the contemplated use temperature; i. e. of the order of 1350 F., or higher.

In accordance with the present invention, the creep rupture strength of the base composition is very substantially increased by adding thereto Cb-Ta from 1.00%- 350% and Cu from 2.00% to 3.00%. The invention alloy may be classed generally as a 15Cr-15Ni-2.5Cu- 2.5Cb-Ta steel alloy.

In the drawing, the single figure is a graphical comparison, at 1350 F., of the creep rupture strength of the invention alloy and an AISI Type 304 18Cr-8Ni steel alloy.

In the invention alloy, the chromium content selected had to be high enough to insure adequate resistance to oxidation and scaling at a contemplated maximum use temperature of 1350 F. to 1450 F., and low enough to inhibit or minimize the formation of embrittling sigma phase. A chromium content of 15% to 17% is suitable for effecting these results. It is advisable to hold the chromium content on the low side since chromium, as well as most of the other elements available for strengthening the base composition, is a ferrite former promoting the weak, body-centered cubic lattice structure which has to be compensated by suitably increased additions of the relatively expensive, and strategically important, austenite forming nickel.

With a chromium content of 15% to 17%, a nickel content of 15% is sufiicient to neutralize the ferrite forming tendencies of chromium and the precipitate producing and strengthening additions. A chromium content of 15 to 17% with a nickel content of about 15% is advantageous from the standpoint of creep strength, as investigations demonstrate that an increase in nickel content above 8% and 10%, required to achieve a fully austenitic structure in a 20% chrome-iron alloy, does not improve the creep strength in any significant degree,

Similarly, if the nickel content in such an alloy is held to 25% does not add materially to the creep strength.

The carbon content of the invention alloy is carefully selected to assure a high enough carbon content for strengthening the alloy through formation of complex carbides yet not so high as to affect adversely forgeability and lead to seams in tubing formed from the alloy. For this purpose, the carbon content has a maximum of 0.12% and preferably is held between 0.03% and 0.05%.

Manganese has a beneficial effect upon hot working properties due to its action upon oxygen and sulphur. It is also desirable as an ingredient due to its tendency to form austenite, although its potency, in this respect, is inferior to that of nickel. Hence, the preferred manganese content is 1.75%, which is near the upper end of the range of manganese commonly found in 18-8 type alloys.

Silicon is a strong ferrite former, and should be kept at a low value where it is desired to promote austenite formation. On the other hand, silicon participates in the formation of strengthening compounds, such as silicides, with columbium and tantalum. It is also a powerful deoxidizer, and enhances resistance to oxidation, at high temperatures, by forming a tightly adherent protective scale, being much more effective than chromium in this respect. By combining a low-range chromium content with a high-range silicon content, satisfactory scaling resistance, with a minimum tendency to sigma-phase embrittlement, is assured. For these reasons, a silicon content of substantially 0.75% is preferred.

As stated, the base composition is strengthened by aly additions designed to produce age hardening processes. For this purpose, the invention alloy, including such additions, is solution heat treated at a high temperature, such as 2200 F. to 2300 F., followed by an aging treatment, or by use, at a lower temperature, such as 1350 F. With suitable alloy additions, a fine dispersion of precipitated compounds in the lattice structure of the matrix is achieved. This fine dispersion resists or retards plastic deformation under stress at elevated temperatures, and thus produces high load carrying ability at such elevated temperatures.

In accordance with the present invention, it has been found that, of available strengthening alloy additions, tantalum or columbium tantalum, together with copper, are most potent in improving the rupture strength at elevated temperatures of the base composition. It also appears that pure columbium, in this combination, is not as effective as tantalum and columbium-tantalum.

Within the composition range previously tabulated, a

preferred composition of a forgeable, high-strength-athigh-temperature alloy embodying the invention, and which is also lean in alloy content and economically practical for superheater tubing, is as follows:

Balance iron with the usual impurities.

The preferred Cb-Ta percentage is 2.50% and the preferred Cu percentage is 2.50%

Alloys of substantially this preferred composition have been compounded, solution heat-treated at 2200 F.- 2300 F. and aged at temperatures of the order of 1300 F.-1500 F. These alloys have then been subjected to stress rupture tests at 1350" F. up to 5,000 hours.- In the accompanying drawing, the stress-rupture curve, plotted on a logarithmic scale withrupture strength in p. s. i. as ordinates and hours under strength as abscissae, is given with actual valuesup to the 10,000 hours point, and extrapolated to 100,000 hours, the test temperature being 1350 F.

Curve A represents the stress-rupture values of alloys embodying the invention, while curve B represents the stress rupture values of an AISI Type 304 18Cr8Ni alloy. It will be observed that, at 1000 hours, the stressrupture strength of the invention alloys is substantially 18,500 p. s. i., over twice that of the Type 304 alloy. At 10,000 hours, the stress-rupture strength of the invention alloys is about 13,700 p. s. i., as compared to about 4700 p. s. i. for the Type 304 alloy. At 100,000 hours, the indicated rupture strength of the invention alloys is about 10,000 p. s. i., as compared to 2700 p. s. i. for the Type 304 alloy. The values for the AISI Type 304 alloy are taken from ASTM-ASME Spec. Tec. Pub]. No. 124.

It will be noted that the invention alloys have stressrupture strengths, at 1350 F., over twice that of the Type 304 alloy. Consequently, they can be used to form tubing suitable for prolonged service at such temperature and having reduced wall thicknesses acceptable from the fabrication, thermal stress gradient, and economy standpoints, while having a prolonged stress resistance at least equal to that of tubing formed from the Type 304 alloys.

The alloys embodying the invention and represented by curve A of the drawing have percentage compositions Within the following ranges:

Percent Cr 15.00-15.25 Ni 15.00 C 0.07- 0.10 Mn 1.75 Si 0.75 Cb-Ta 3.00 Cu 3.00

Balance iron with the usual impurities.

Percent Cr 15.00-20.00 Ni 12.00-18.00 C 0.02-0.15 Mn 0.25-2.50 Si 0.10-1.00 Ta(Cb-Ta) 1.00-3.50 Cu 2.003.00

Balance iron with the usual impurities.

2. As an article of manufacture, a tube fabricated from the alloy defined in claim 1.

3. A forgeable austenitic steel alloy having superior stress resistance and corrosion resistance properties, and

freedom from impact embrittlement, in extended service under stress at temperatures of the order of 1300 F.; said alloy having the following composition:

Balance iron with the usual impurities.

4. As an article of manufacture, a tube fabricated from the alloy defined in claim 3.

5. A forgeable austenitic steel alloy having superior stress resistance and corrosion resistance properties, and freedom from impact embrittlement, in extended service 5 under stress at temperatures of the order of 1300' F.; said alloy having the following composition:

Cr 17.00% maximum. Ni 15.00% maximum. C 0.12% maximum. Mn 2.00% maximum. Si 0.75% maximum. Ta(Cb-Ta) 2.5%.

Balance iron with the usual impurities.

stress resistance and corrosion resistance properties, and 15 freedom from impact embrittlement, in extended service under stress at temperatures of the order of 1300 F.; said alloy having the following composition:

Percent Cr l5.0015.25

Ni 15.00 C ().07-0. 10 Mn 1.75 Si 0.75

Ta(Cb-Ta) 3.00 Cu 3.00

10 Balance iron with the usual impurities.

8. As an article of manufacture, a tube fabricated from the alloy defined in claim 7.

References Cited in the file of this patent UNITED STATES PATENTS 2,540,509 Clarke Feb. 6, 1951 

2. A FORGEABLE AUSTENITIC STEEL ALLOY HAVING SUPERIOR STRESS RESISTANCE AND CORROSION RESISTANCE PROPERTIES AND FREEDOM FROM IMPACT EMBITTLEMENT, IN EXTENDED SERVICE UNDER STRESS AT TEMPERATURES OF THE ORDER OF 1300*F.; SAID ALLOY HAVING THE FOLLOWING COMPOSITION: 